Northbrae rhyolite

Berkeley is full of interesting rocks, many of them preserved in pocket parks. The most prominent of these is Indian Rock.

Click the photo for a 1000-pixel version. Some 100 years ago this tough rhyolite, a volcanic rock very high in silica, was mapped as part of the same rock body as the one in Leona Quarry and farther down along the hills. But a master’s student at Cal State Hayward gave it a good look in the 1990s and determined that the Northbrae rhyolite, as this occurrence was named, is quite a different rock. (In fact the Leona rhyolite isn’t considered a rhyolite any more, but rather a high-silica welded tuff/volcaniclastic sequence.) Whereas the first mappers thought that both rocks were Pliocene, which is quite young (about 5 million years), the Leona was later shown to be Jurassic (about 150 m.y.). The Northbrae is not that old, but neither is it as young as Pliocene. It’s just a little older than the Moraga Formation basalt and the volcanic rocks of Sibley volcano, about 11 million years, making it Miocene. It came out in that same episode of eruptions, which today sits to the north around Clear Lake and The Geysers.

And it’s still definitely rhyolite. Rhyolite is the stiff, slow-moving lava that makes up little volcanoes like the young dome inside Mount St. Helens, or the rugged knobs of the Inyo Domes, over the Sierra in the Mammoth Lakes area, or farther south in the Coso Range. It makes great rock for climbers—strong, imperishable, full of handholds and rarely giving way under a person’s weight.

The rhyolite of Berkeley is well worth a visit. Just go at a time when the climbers aren’t busy; the rock parks swarm with them in nice weather. I don’t think Oakland has any of this rock, but it might.

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This entry was posted on 27 December 2010 at 10:40 pm and is filed under oakland rocks, other. You can follow any responses to this entry through the RSS 2.0 feed.
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As a rock climber interested in the origin of the things we climb on I have long wondered about Indian Rock. Lots of debate about it among the climbers…We thought volcanic, possibly reworked and indurated chert , or maybe Monterey Formation. I’m talking about 40 years of speculation!
Volcanic made sense because of nearby Mortar Rock being obviously so. Other outcrops in the area look like Indian Rock does, too. For myself, I thought tuff. Guess I was wrong!
Indian Rock bouldering nurtured a generation or two of fine rock climbers who went on to make great ascents in Yosemite and beyond. Thanks Andrew for the update!